1. Technical Field
The present invention relates to a structure of a solid oxide fuel cell.
2. Description of the Related Art
As environmental pollution gradually increases, global warming problems are becoming more serious. For this reason, the Kyoto protocol was adopted in 1997 in order to set guidelines for the reduction of carbon dioxide emission and to handle energy-related environmental problems in earnest. Thus, fuel cell technology which exhibits high cell performance and is environmentally friendly is receiving renewed attention.
A fuel cell is a device for directly converting the chemical energy of fuel (hydrogen, LNG, LPG, etc.) and air into electric power and heat using an electrochemical reaction. Unlike conventional techniques for generating power including combusting fuel, generating steam, driving a turbine and driving a power generator, the fuel cell neither undergoes a combustion procedure nor requires an operator and is thus regarded as a novel power generation technique which results in high cell performance without being accompanied by any concomitant environmental problems. The fuel cell discharges very small amounts of air pollutants such as SOx and NOx and also generates a small amount of carbon dioxide and is thus a pollution-free power generator, and is furthermore advantageous in terms of producing very little noise and not causing any vibrations.
The fuel cell includes for example a phosphoric acid fuel cell (PAFC), an alkaline fuel cell (AFC), a polymer electrolyte membrane fuel cell (PEMFC), a direct methanol fuel cell (DMFC), a solid oxide fuel cell (SOFC) and so on. In particular, the SOFC exhibits high power generation efficiency because of low overvoltage based on activation polarization and low irreversible loss. Furthermore, the SOFC is advantageous because various types of fuel, such as hydrogen, carbon and a hydrocarbon, may be used, and also because the reaction rate at the electrodes is high, thus obviating the need to use an expensive noble metal as an electrode catalyst. Moreover, the temperature of the heat generated during power generation is very high, and thus the heat is very usable. In addition, heat generated from the SOFC is used to reform fuel and may also be utilized as an energy source for industrial purposes or for air cooling in a cogeneration system. Hence, the SOFC is essential for realizing the hydrogen-based society of the future.
In accordance with the operating principle of the SOFC, the SOFC typically generates power through the oxidation of hydrogen or carbon monoxide, and the reactions at the anode and cathode are represented by Reaction 1 below.
Reaction 1
Anode:H2+O2−→H2O+2e CO+O2−→CO2+2e 
Cathode:O2+4e→2O2−
Overall Reaction:H2+1/2O2→H2O
In the above reactions, electrons are delivered to the cathode through an external circuit, and simultaneously the oxygen ion generated at the cathode is transferred to the anode through an electrolyte. At the anode, hydrogen or carbon monoxide is combined with the oxygen ion, thus producing electrons and water or carbon dioxide.
Many attempts have been made to improve the structure of the SOFC in order to inhibit the oxidation of the material to thereby more efficiently exhibit cell performance over a long period of time. Examples of the structure of a conventional SOFC include a planar type, a tubular type, a flat tubular type, a honeycomb type, a delta type, etc. The structures thereof are classified into a cathode-supported structure and an anode-supported structure depending on whether the support is made of a cathode material or an anode material.
In the case of a general cathode-supported tubular fuel cell, while air flows through the inside of the tubular cell and fuel flows around the outside of the tubular cell, an electrochemical reaction ensues. Such a cathode-supported tubular fuel cell is problematic in that the cathode material is expensive, but is advantageous because the outside atmosphere of the tubular cell is a hydrogen atmosphere and thus a current collector such as a nickel pelt, an interconnector and so on may not be oxidized, resulting in extended cell lifespan and reliability.
However, when a support is made using a comparatively inexpensive cathode material, hydrogen fuel passes through the inside of the tubular cell and air flows around the outside of the tubular cell, so that a current collection material outside the tubular cell is oxidized when the cell is operated for a long time. In order to prevent this problem, an expensive current collection material is required.
Japanese Unexamined Patent Publication No. 2003-297388 discloses a fuel cell structure in which an electrode is disposed inside the tubular cell and a plurality of holes is formed in an anode, thus supplying fuel through the holes. Although this structure may improve the collection of current, it suffers from the above-mentioned oxidation problem and the cell lifespan similar to that of the conventional cell structure.